1. Field of the Invention more specifically to polymers crosslinked with isocyanurates or biurets of diisocyanates. The present invention pertains especially to automotive clearcoats comprising polymers crosslinked with isocyanurates or biurets of diisocyanates.
2. Background of the Invention and Material Information
In the field of automotive coatings, it has become an objective to obtain a clearcoat (i.e. the outermost automotive coating) which is resistant to being etched by environmental fallout. Environmental etching is manifested in the form of pitting and/or waterspotting and/or chemical spotting of the coating. Etch resistance is desirable because it improves the appearance and useful life of the coating.
In general, etch resistance is measured by visual examination of the coating, although it can also be measured by a profilometer or by subjecting a coating on a test panel to a saline solution in a temperature gradient oven test. That is, etch resistance is generally measured by visual examination by individuals skilled, at examining finishes for the degree of etch therein. Etch resistance has become more important as the amount of acid rain, and other industrial fallout, has increased. Currently, environmental etching of automotive finishes is generally greatest in the areas most greatly affected by industrial fallout.
In addition to testing for resistance to environmental etching, automotive coatings are typically tested by exposure to: (1) ultraviolet radiation emitted from a ultraviolet emitting light bulb, (2) ultraviolet radiation emitted from a xenon arc bulb, (3) exposure to sunlight in regions differing in latitude, climate, and pollutant levels, (4) high relative humidity at high temperature, (5) impacts made by small, hard objects, etc.
Accordingly, it is most preferable to produce a coating having a optimum mix of characteristics with regard to all of the tests described above, among still additional tests. If a coating exhibits excellent resistance to environmental etching, but quickly degrades upon being subjected to ultraviolet radiation, the coating is unlikely to be as commercially successful as a coating which has better resistance to ultraviolet radiation while having acceptable resistance to environmental etching. It is the sum of all of the characteristics of any particular coating which determine if it is valuable in the real world of automotive coatings.
Furthermore, in the manufacture and use of automotive coating formulations, it is desirable to: (1) select compounds in order to minimize environmental impact and/or the expense involved in recovery thereof, and (2) utilize compounds which do not present an unacceptable risk to occupational safety or occupational health.
Currently the most prevalent clearcoat compositions utilized in the automotive industry are one-component melamine cure clearcoat compositions. These compositions do not require that two or more reactive components be mixed immediately before being applied to a substrate to be coated. Rather, these compositions crosslink upon exposure to heat in the presence of catalysts, and can be applied as one component compositions because they do not react until they are exposed to relatively high temperatures in the presence of a suitable catalyst.
However, one component compositions which comprise melamine are not without their disadvantages. First, it has been found that cured automotive clearcoats made using these compositions exhibit unsatisfactory environmental etch characteristics. Second, melamine systems split off a byproduct upon crosslinking. This by-product is usually a low molecular weight organic solvent, which adds to the volatile organic compounds (i.e. VOC's) which are released (or captured) during the coating process. Third, the small organic compounds released may become involved in undesirable reactions with other components in the composition or with components present in adjacent coatings.
One component, blocked coating compositions comprising blocked isocyanate functionalities are also known in the area of automotive coatings. These one component compositions have a blocked isocyanate functionality (e.g. a methyl ethyl ketoxime blocked isocyanurate of hexamethylene diisocyanate) in combination with a film forming polymer comprising at least one functional group reactive with an isocyanate functionality. The crosslinker comprising the blocked isocyanate functionality is combined with the polymer and a liquid carrier in which the crosslinker and polymer dissolve and/or disperse. The resulting mixture must be stored at a temperature below the deblocking temperature, i.e. so that the crosslinker cannot react with the polymer. The one component mixture can be sprayed onto a substrate so that a film of the composition is formed. The film is thereafter heated in an oven so that the blocking agent "deblocks" (i.e. is released), whereby the isocyanate is free to react with the polymer. The reaction of the isocyanate with the polymer results in the production of a crosslinked polymer network, i.e. a cured coating.
One component compositions comprising a blocked isocyanate also exhibit several disadvantages. First, the presence of the blocking agent produces the need to utilize proportionally more solvent dispersant, because the blocking agent enhances the size of the crosslinker, requiring more solvent or dispersant. Second, upon deblocking, the blocking agent is volatilized, this increasing the volatile organic compounds (VOC) present in the coating, when compared to the unblocked two component composition. Third, the release of the blocking agent in one component compositions places a mobile species into the film. The presence of this mobile blocking agent can be a detriment, since it could potentially react with other species present in the film or in an adjacent uncured film layer which may be also present. Fourth, the deblocking agent represents a cost which must be borne by the manufacturer and user of the blocked isocyanate composition. Finally, the use of a blocking agent requires that the composition be brought to a higher temperature than is required for the curing of two component compositions. The use of higher temperatures for the curing operation is undesirable because it requires the input of greater energy (i.e. it is expensive), and because it can result in deformation of plastic automotive body panels during the curing step.
Although two component coating compositions have also been utilized in the art of automotive coatings, two component compositions, until today, have not been as preferred as one component systems, for several reasons. First, two component compositions which comprise isocyanurates present the possibility that persons involved in the process of making and applying the composition will be exposed to an isocyanurate in its reactive form. Isocyanurates are known to be skin irritants. Second, it is necessary to keep the two components (i.e. the crosslinker and the reactive polymer) apart until such time as they are applied to a substrate, because upon contact with one another they react to form a crosslinked network.
The two component isocyanate-based compositions known to have been used in the art of automotive coatings are compositions which comprise an isocyanurate of hexamethylene diisocyanate as the only isocyanate functional crosslinking agent therein. The isocyanurate of hexamethylene diisocyanate is known to be advantageous for this use because it produces a coating having a good hardness to flexibility ratio. The isocyanurate of hexamethylene diisocyanate is on of few isocyanurates which is soluble or dispersable in the solvents and dispersants used in making coatings. Furthermore, the isocyanurate of hexamethylene diisocyanate is a relatively inexpensive component, in comparison with other isocyanurates.